Kelly O'Hair over at Sun, writes about the current composition and integration process for the Open JDK project.
Sorry Dorothy, we aren't in Kansas anymore, and there isn't just one repository anymore. ;^) The JDK team has been using TeamWare (also a Distributed SCM like Mercurial) for a very long time, and the strategy adopted involves having different teams (usually based on functionality) push changes through specific team areas rather than everyone integrating into one MASTER area. Each team can focus their testing on the changes their team is making, and also protect themselves from regressions made by other teams. It also allows for changes to be "baked" before being pushed into the MASTER area. There is some overhead here, in that an assigned integrator for each team will need to periodically sync up or merge with the MASTER area, test the merge, and push the resulting merge up to the MASTER area.
Read Kelly's complete post: http://blogs.sun.com/kto/entry/openjdk_mercurial_wheel